1. Field of the Invention
This invention relates to a method for preparing a contact mass for use in the production of alkylhalosilanes by the direct process and a process for producing alkylhalosilanes using the contact mass. More particularly, it relates to a process for continuously producing alkylhalosilanes by effecting gas-solid contact reaction between metallic silicon powder and an alkyl halide in the presence of a copper catalyst.
2. Prior Art
With respect to the synthesis of alkylhalosilanes, Rochow first disclosed in U.S. Pat. No. 2,380,995 direct synthesis reaction between metallic silicon and alkyl halides in the presence of copper catalysts. Since then, there have been reported a number of research works relating to various co-catalysts used together with copper catalysts, various copper catalysts and treatment thereof, reactors, additives used during reaction, and the like.
The direct synthesis process involves activating a contact mass comprising metallic silicon and a copper catalyst and introducing an alkyl halide into the activated contact mass for accomplishing direct gas-solid contact between metallic silicon and alkyl halide, thereby producing alkylhalosilanes. The period required to activate the contact mass is defined by R. J. H. Voorhoeve, Organohalosilanes, 1967, pp. 250-251, as a time taken until a phase of Cu.sub.3 Si active in direct synthetic reaction (.eta. phase) is formed in the contact mass. This is generally known as an "induction period." Various methods have been proposed for reducing the induction period.
JP-B 43400/1988, for example, discloses an alkylhalosilane producing method involving charging a reactor with silicon, catalyst and co-catalyst, passing nitrogen through the reactor, heating the reactor up to 200.degree. C., admitting methyl chloride, heating the reactor up to 345.degree. C., maintaining the temperature for 1 hour, and thereafter, cooling the reactor to 330.degree. C. This method allows a larger amount of methyl chloride to be admitted. It is described that the reaction mass should be treated above 340.degree. C. for about 20 to 30 minutes when reaction is done at a temperature below 340.degree. C., and that such treatment is unnecessary when reaction is done at a temperature above 340.degree. C. or when copper chloride is used.
JP-A 78390/1979 discloses the steps of commencing reaction at 360.degree. C. in order to reduce the induction period, lowering the temperature by 10.degree. C. in each of initial 2-hour periods and thereafter, lowering the temperature at a rate of 15.degree. C./hr. until 280.degree. C. is reached.
According to these methods, the contact mass is once heated to a temperature above the steady reaction temperature, and the temperature is then lowered for reaction to continue. Once heated to the elevated temperature, however, the copper catalyst is spent and likely to lower its catalytic activity at lower temperatures. It is also known that copper and similar solid catalysts become sintered owing to the thermal hysteresis at elevated temperatures. With the progress of sintering, the catalyst life becomes shorter and the yield of the desired dialkyldihalosilane becomes lower.
The procedure of once heating to a higher temperature and then cooling to the steady reaction temperature is undesirable from the energy standpoint partly because the reaction of alkyl halide with metallic silicon is exothermic. This is disadvantageous especially in the case of commercial scale reactors. Additionally, the cooling step takes a long time. If a long time is passed until the steady reaction temperature is reached, there is a likelihood of failure to activate to the catalyst.
JP-A 187933/1986 discloses another alkylhalosilane producing method involving passing nitrogen through a reactor charged with metallic silicon powder and a copper catalyst for fluidization, heating the reactor at 325.degree. C., passing HCl gas for 75 minutes to produce trichlorosilane, thereafter charging the reactor with an accelerator, and feeding methyl chloride for reaction to continue. This method, however, uses HCl as one reactant, on account of which tetrachlorosilane and trichlorosilane form. These chlorosilanes are generally difficult to separate from other useful silanes.
Furthermore, JP-B 40035/1989 discloses pretreatment by heating a mixture of metallic silicon powder, catalyst, and co-catalyst to 300.degree. C. and then introducing an equimolar mixture of dimethyldichlorosilane and methyl chloride. This method, however, is less effective to activate the catalyst. Even if the reaction is activated, the dialkyldihalosilane used in the pretreatment goes to waste.
In summary, prior art alkylhalosilane producing methods suffer from the following problems associated with the activation of the contact mass. (1) The copper catalyst can be sintered during long-term thermal hysteresis at elevated temperatures, resulting in the catalyst losing lifetime and selectivity. (2) Excessive heating is disadvantageous from the energy standpoint. (3) A longer induction period fails to activate. (4) Extra HCl is used with formation of by-products which are difficult to separate, leading to a lowering of selectivity.